Much attention has been focused on the utilization of an excimer laser as a light source of a reduced projection exposer for fabrication of semiconductor devices. This is because the utilization of the excimer laser as such a light source can expect many excellent advantages including the limit wavelength of exposure light reduced to below 0.5 .mu.m because of the short wavelength of the excimer laser (the wavelength of a KrF laser being about 248.4 nm), a focus depth larger than that for the g or i rays of a prior art mercury-vapor lamp so long as a resolution power is identical, a small lens numerical) aperture (NA), an enlarged light exposure area, and a high output power.
However, the excimer laser has two big problems when used as the light source of the reduced projection exposer.
One of the problems is that, since the output light wavelength of the excimer laser is as short as 248.35 nm, this limits the materials through which this wavelength of light can pass, to only quartz, CaF.sub.2, and MgF.sub.2, and further among these materials, only quartz can be used as the material of the lens, when taking its homogeneity and machining accuracy into consideration. For this reason, it becomes impossible to design the reduced projection lens subjected to a chromatic aberration correction. Accordingly, this requires the band of the excimer laser to be narrowed to such an extent that the chromatic aberration is negligible.
The other problem is to suppress an undesired speckle pattern developed by narrowing the band of the excimer laser and also to avoid reduction in the laser power caused by the narrowed band.
One of techniques for narrowing the band of an excimer laser is known as the injection lock system. In the injection lock system, wavelength selective elements (such as etalons, diffraction gratings or prisms) are disposed with a cavity at an oscillator stage so that pin holes limits its spatial modes, the laser oscillates in a single mode, and the oscillated laser beam is synchronously injected into its amplification state. For this reason, the output laser beam is highly coherent and thus when such output beam is used as the light source of the reduced projection exposer, an undesired speckle pattern takes place. Generally speaking, the generation of a speckle pattern is considered to depend on the number of spatial transversal modes contained in the laser beam. More specifically, it is known that a smaller number of spatical transversal modes included in the laser beam tends to more develop a speckle pattern, whereas a larger number of spatial transversal modes tends to less develope a speckle pattern. The aforementioned injection lock system, which is a technique for achieving a narrow band essentially by remarkably decreasing the number of spatial transversal modes, cannot be employed in the reduced projection exposer, since it involves the development of a speckle pattern as a big problem.
There is another promising technique for narrowing the band of an excimer laser which uses etalons. As a prior art of such etalon-based techniques, there has been proposed such a technique from the AT & T Bell Laboratory that etalons are disposed between a front mirror and a laser chamber in an excimer laser to realize the narrowed band of the excimer laser. This system, however, has had problems and defects that a spectrum line width cannot be narrowed satisfactorily, a power loss due to the etalon insertion is large, and the number of spatial transversal mode cannot be made considerably large.
To solve the above problems, the inventors of the present application provide such an arrangement that wavelength selective elements having a large effective diameter (about several ten mm) such as etalons are interposed between a rear mirror and a laser chamber in an excimer laser so that the uniform narrow band of the laser is about below 0.003 nm in full width at half maximum in output beam of about 50 mJ per pulse. That is, when there is employed such an arrangement of the etalons disposed between the rear mirror and laser chamber of the excimer laser, the essential conditions of the laser required to be used as the light source of a reduced projection exposer, that is, the problems or requirements of narrowing the band of the laser, positively acquiring the necessary number of spatial transversal modes and minimizing the power loss caused by the etalon insertion can be solved.
The arrangement of the wavelength selective elements disposed between the rear mirror and laser chamber of the excimer laser is advantageous in that the band of the laser can be narrowed, the necessary number of spatial transversal modes can be positively acquired and the power loss caused by the etalon insertion can be reduced, but defective in that the above solving means cause variations in the center wavelength of an oscillated output beam of the laser, the oscillation of the laser at multiple wavelengths or the remarkable reduction of laser power. This tendency becomes remarkable, in particular, when two or more wavelength selective elements having different free spectral ranges are employed for the purpose of narrowing the laser band.
For eliminating the above disadvantages, the inventors of the present application suggest a laser control method in which the center wavelength and output power of a laser is stabilized by executing the following three controls at the same time or alternately.
(1) Center wavelength control
Shifts the transmission wavelength of one of the wavelength selective elements at least having a minimum free spectral range to thereby make the center wavelength of the laser output coincide with a desired value.
(2) Overlapping control
Shifts respectively the transmission center wavelengths of ones of the wavelength selective elements other than the element having the minimum free spectral range to overlap the transmission center wavelengths of all the wavelength selective elements and thereby obtain a maximum laser output.
(3) Power control
Controls a voltage applied to electrodes provided within the laser chamber in such a manner that the laser output power has a desired level.
Meanwhile, the properties of a laser medium gas used in an excimer laser gradually deteriorate with time, for which reason it becomes difficult to obtain a desired output power by means of only the power control based on the control of the electrode application voltage. To remove such difficulty, in the present invention, when the electrode application voltage exceeds the desired level, the components of the laser gas are controlled, that is, partial laser gas replacement is carried out. However, the execution of the partial laser gas replacement causes the gradual increase of the laser output power. For the purpose of suppressing such increase in the laser output power, the above power control is executed. In this connection, since an increase in the laser output power caused by the partial laser gas replacement is a different sort of phenomenon from stationary-mode output variations, the stationary-mode power control cannot cope properly with the output power increase.
In addition, when the partial laser gas replacement is carried out, the laser is subjected to the influence of the partial gas replacement for a considerably long period of time, during which the laser cannot produce a stable output power by means of the stationary-mode control.
It is therefore a first object of the present invention to provide a method of controlling a narrow-band oscillation excimer laser which can reliably follow a power variation caused by the above partial laser gas replacement and can produce a stable laser output power.
With such a narrow-band excimer laser, the overlapped condition of the etalons is improper, in general, at the time of initiating the laser and thus the laser output is very low, which may sometimes result in that the laser oscillation cannot be achieved or such oscillation can be attained only with a power level lower than the laser power and wavelength detection limit of a power monitor, thus disabling the detection of the laser power and wavelength.
For this reason, it is impossible to accurately detect the center wavelength of an output beam of the laser at the time of activating or initiating the laser, and therefore it is difficult to first perform the center wavelength control of causing the center wavelength of the laser output beam to coincide with a desired value.
Accordingly, a second object of the present invention is to provide a system of controlling a narrow-band excimer laser which can quickly control the laser at the time of initiating the laser.